Fluoride Exposure Affects Thyroid Hormones

A review of human clinical studies suggests that high fluoride exposure may negatively impact thyroid function and increase the risk of thyroid disease, particularly in children. Fluoride is commonly added to public water supplies in many countries to prevent tooth decay, but concerns have been raised about its potential effects on other aspects of health, including the endocrine system. The thyroid gland—which regulates metabolism, growth, and development—is known to be sensitive to environmental factors, and disrupted thyroid function can have widespread effects, especially in developing children.

The researchers systematically analyzed data from 27 studies, most of which were conducted in Asia and focused on children. These studies assessed fluoride exposure by measuring its concentrations in drinking water, urine, blood, or dietary intake. They assessed thyroid health through hormone levels, disease diagnoses, or thyroid size.

In 24 of the studies, thyroid function was evaluated by measuring levels of hormones in the blood, including thyroid-stimulating hormone (TSH), which helps regulate thyroid activity. When researchers compared people with the highest fluoride exposure to those with the lowest, they found that the high‑exposure group had average TSH levels that were 1.05 μIU/mL higher than the low-exposure group. However, this increased hormonal activity in the thyroid gland did not always correspond with thyroid disease and often stayed within normal reference ranges.

The data from the studies showed that TSH levels stayed about the same at low fluoride levels, but they began to rise steadily once fluoride in drinking water went above roughly 2.5 milligrams per liter (mg/L). The World Health Organization recommends fluoride in drinking level at a concentration of 0.5 to 1.5 mg/L. The optimal fluoride concentration in American public drinking water systems is 0.7 mg/L, according to the U.S. Public Health Service, but the current U.S. Enivornmental Protection Agency standards outline a maximum contaminant level of 4.0 mg/L. The studies included in this review were mostly conducted in Asia, where water fluoride levels vary widely: In China, for example, fluoride may exist in higher concentrations due to natural occurrence or industrial contamination. Fluoride exposure can vary widely according to climate, local environment, pollution, and the sources of freshwater.

The researchers noted that fluoride exposure above a certain level appears to correlate with increase TSH levels, which indicates altered thyroid gland function. However, the connection between fluoride and other thyroid hormones was less clear. One hormone, triiodothyronine (T3), showed some evidence of decreasing as fluoride exposure increased.

Though fewer in number, the studies that looked at thyroid disease outcomes found possible links between high fluoride exposure and conditions such as goiter and hypothyroidism. These associations were observed in both children and adults.

Overall, the findings suggest that exposure to high levels of fluoride in drinking water may disrupt normal thyroid function, especially by increasing TSH levels, and may raise the risk of some thyroid-related diseases.